They reduce car crashes with other cars. I don't know off the top of my head if they reduce car-pedestrian crashes but I suspect not.

I used to live off of a busy roundabout in a place that makes heavy heavy use of roundabouts (almost no four-way stops). It felt dangerous as fuck honestly. The crosswalks are a little "downstream" of the true circle, where cars have already begun to exit. Frequently (saw this a few times myself) a car would stop for a pedestrian and then get rear-ended by another car focused on exiting the roundabout.

This was not the US so unfamiliarity with roundabouts can't be blamed. They were the norm there.